The present invention relates to improvements in the process disclosed in U.S. Pat. No. 3,835,214, Hurst et al., assigned to the U.S. Atomic Energy Commission. As explained therein, the phosphoric acid by-product of the wet-process for producing phosphate fertilizer from uraniferous phosphate ores contains enough uranium to warrant the development of a profitable process for its recovery. Such a process must include the recycing of the substantialy uranium-free phosphoric acid to the wet-process plant in an acceptable degree of purity.
Briefly, the process of the patent comprises two liquid-liquid solvent extraction circuits for the recovery of uranium in the uranyl form as ammonium uranyl tricarbonate from which product uranium oxide is recovered by calcining. In the first circuit the uranium from the feed solution is reduced to the uranous form, contacted with an ion exchange agent selective for uranous uranium consisting of a mixture of mono- and di- (alkyl-phenyl esters of orthophosphoric acid (OPPA) dissolved in a suitable solvent, and the loaded agent is subjected to an oxidative strip with phosphoric acid and an oxidizing agent. In the second circuit the strip solution containing the uranium in uranyl form is extracted with the liquid-liquid sovent extraction agent di(2-ethylhexyl)phosphoric acid (DEHPA) to which has been added a synergist, trioctylphosphine oxide (TOPO), dissolved in an organic diluent. The loaded agent is scrubbed with water and stripped with ammonium carbonate to precipitate the uranium as ammonium uranyl carbonate from which the uranium oxide is recovered.
It has been found that processes like that disclosed in the above patent are subject to a number of disadvantages. The most feasible solvent extraction agent for recovering the uranium after it has been reduced to uranous form is a mixture of the mono- and di- (alkyl-phenyl) esters of orthophosphoric acid and it is well known that the ratio of these esters to each other in this process must be maintained within a required range. It has been found that when the prior process is run continuously, more of the monosubstituted ester is consumed than the disubstituted ester from time to time. It has also been observed that when the uranium-free phosphoric acid is returned to the wet-process plant without removal of the uranium process organics from the raffinate, a striking deterioration of rubber elements in the plant equipment, particularly the rubber lining of the plant evaporators, occurs because of the presence of these organics. The term "process organics" means organic substances added to the system during the process, rather than those in the feed material, e.g., the OPPA esters and organic diluent.
In the conventional process, after the uranium in uranous form is recovered on the agent it is subjected to an oxidative strip using evaporated phosphoric acid as a stripping agent and an oxidizing agent such as sodium thiosulfate, chlorine, or sodium chlorate. The difficulty with these oxidizing agent is that they introduce chemical impurities into the system, such as sodium, sulfur and chlorine, which are impossible to feasibly remove and the final results are contamination of phosphoric acid returned to the wet-process plant and contamination of the uranium oxide product.
It is also noted that water scrubbing the DEHPA-TOPO agent in the second circuit before stripping does not adequately remove such impurities as phosphorus, iron, and vanadium. Use of an ammonium carbonate strip of the oxidized uranium from the agent results in the precipitation of uranium in the strip concentrate solution and introduces the problem of organic wetting of the yellow cake product.
Recycling of the stripped organic of the second circuit without regeneration presents phase separation problems resulting from emulsions formed with sodium, aluminum, and silicon.
Accordingly, it is an object of this invention to provide a process for the recovery of uranium from a wet-process type phosphoric acid solution using two liquid-liquid solvent extraction circuits as discussed above in which the uranium process organics in the raffinate from the first circuit are substantially removed from the raffinate before recycling to the wet-process plant. The uranium process organics are returned to the first circuit thereby maintaining the required balance between the mono and disubstituted esters.
It is another object of this invention to provide a process as stated in which an oxidative agent is used with phosphoric acid in the first circuit strip which does not add contaminating materials to the system, nor form precipitates with other reactants in the evaporated phosphoric acid.
It is still another object of this invention to provide a process as stated in which a more effective scrubbing agent for the loaded organic in the second circuit is provided to remove contaminating impurities of the uranium oxide product.
It is a further object of this invention to provide a process as stated in which emulsions occurring from recycling of the liquid-liquid solvent extraction agent in the second circuit are prevented.
It is still a further object of this invention to provide a process as stated in which the stripping agent used in the second-circuit strip produces a totally soluble strip concentrate solution which can be treated in an external system for the precipitation of uranium thereby eliminating any problem of organic wetting of the yellow cake product.